toolkit: no thread panic in http_fetch, no casts, exact egress-proxy form

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/toolkit/src/input.rs

@@ -33,7 +33,9 @@ impl From<std::io::Error> for InputError {
 /// (`Read::take`); more than MAX_INPUT is TooLarge.
 pub fn read_input(stdin: &mut dyn std::io::Read) -> Result<String, InputError> {
     let mut buf = Vec::new();
-    let n = stdin.take(MAX_INPUT as u64 + 1).read_to_end(&mut buf)?;
+    let n = stdin
+        .take(u64::try_from(MAX_INPUT).map_or(u64::MAX, |n| n.saturating_add(1)))
+        .read_to_end(&mut buf)?;
     if n > MAX_INPUT {
         return Err(InputError::TooLarge);
     }